1. Field of the Invention
The present invention is directed to electrical frequency measurement, and particularly to a signal processing apparatus for providing accurate temporal frequency measurement of multiple time-coincident radio frequency (xe2x80x9cRFxe2x80x9d) signals over a wide instantaneous bandwidth and dynamic range.
2. Description of the Related Art
Electrical frequency measurement is a requirement for numerous signal processing, communication, and signal measurement systems in use today and in the foreseeable future. A wide instantaneous operating bandwidth is essential in many systems. Frequency is a key measurement parameter for the purpose of RF signal tracking, characterization, and identification. Commercial and military applications operating in congested RF environments must process multiple signals simultaneously and provide measurement of selected signals while rejecting other undesirable interfering signals within the operating bandwidth. Present day applications include satellite and terrestrial communications systems, radar receivers, air traffic control radar, and RF test equipment.
Existing equipment implement one or more frequency measurement devices including instantaneous frequency measurement (xe2x80x9cIFMxe2x80x9d) receivers, frequency discriminators, and channelized receivers. Each alternative has drawbacks. For example, most frequency discriminators and IFMs operate within a limited internal input signal dynamic range, typically within the saturation region, and, hence, require additional circuits to constrain input signals to this region. Because these devices are not frequency channelized, they are unable to correctly resolve time-coincident pulse-on-pulse or pulse-on-continuous wave (xe2x80x9cCWxe2x80x9d) signals. These devices are constrained to handle a single signal at a time. If a second signal occurs in the same time window, an IFM cannot measure the second signal. RF channelizer technology alone cannot provide sufficient frequency resolution without incurring a high cost for the required large number of equivalent RF filter bank elements and video circuitry, and suffer from false xe2x80x9crabbit earsxe2x80x9d triggering with such narrow filter widths. In addition, RF channelizers provide only relatively coarse frequency measurements.
Furthermore, because the IFM can measure only one signal at a time, a CW wave (i.e., a signal that is very long in pulse width) can hinder the ability of an IFM to measure the remaining environment. Thus, by driving an IFM with a strong signal that is turned on continuously, the IFM becomes unable to operate and collect other threats in the environment.
A U.S. patent of interest includes U.S. Pat. No. 5,682,238 to Levitt et al. (the subject matter of which is incorporated herein by reference), which discloses a signal processing apparatus with an optical phase measurement processor that provides phase difference measurements of multiple signal inputs.
The present invention measures frequency of multiple, time-coincident (i.e., two or more signals in which a portion of one signal duration coincides in time with a portion of one or more other signal durations) RF signals within a wide instantaneous input bandwidth and dynamic range. In one embodiment of the present invention, RF delay line 104 circuits are combined with a single optical processor 200, a spatial-domain phase measurement technique, and associated algorithms to provide simultaneous fine frequency measurement resolution that is maintained over the entire dynamic range. RF delay line 104 and power splitter 102 components provide two wideband RF inputs for the optical processor 200, one time-delayed relative to the other. Channelizer processing circuitry utilizes optical processor photodetector outputs to perform signal detection, coarse frequency estimation, fine frequency estimation, frequency and phase calibration, and combination of coarse and fine frequency for final output.
Multiple signal inputs of interest are simultaneously detected and measured in frequency, and optionally, other time-domain parameters. Basic frequency measurement occurs through two concurrent processesxe2x80x94detection of signal presence with (1) coarse frequency measurement, and (2) fine frequency measurement. Both functions are performed in a single measurement cycle.
Detection and coarse frequency measurement are achieved through modulator induced optical beam deflection onto a photodetector array 212, wherein active detector positions directly correspond to incident RF signal spectral components. Within a corresponding photodetector element bandwidth, fine frequency is obtained by measurement of relative phase difference between the original RF0 and a delayed version RF1 of the incident RF signal, effectively through frequency to phase translation. This is accomplished by RF modulation of two optical input beams 205 followed by projection of the modulated beams to produce an optical interference pattern upon the photodetector elements. The resulting spatial intensity pattern phase variation is decoded by an efficient spatial sampling technique and associated algorithms, such as those employed in the ""238 patent.
The delay time is selected to achieve a specified fine frequency resolution and provide unambiguous phase measurement within one coarse frequency resolution width. The number of photodetector elements along the frequency deflection axis of the photodetector array 212, instantaneous operating bandwidth, in conjunction with other opto-electronic design parameters determine the effective coarse frequency resolution.
The photodetector outputs are applied to a frequency encoder 400 circuit to correct the raw phase and amplitude (or magnitude) data offsets, detect and locate active signals within the array 212, convert phase to fine frequency, and finally, concatenate coarse to fine frequency for readout. The encoded combination of coarse frequency with fine frequency discriminator output permits precision frequency measurement over the entire system bandwidth. Additionally, multiple time-coincident RF input sources, which are resolvable in frequency, can be processed in parallel using the present invention.
Unlike commercial electronic frequency discriminator and IFM devices, the present invention permits frequency measurement of multiple, time-coincident, and continuous wave signals, which is a shortfall of these single channel devices. The present invention has a separation at the frequency subchannels, and each subchannel performs the frequency discriminator type operation simultaneously. Most frequency discriminators and IFMs function similar to only one subchannel of the present invention, thereby processing the composite input bandwidth without first separating individual RF signals. This often leads to gross frequency measurement errors under time-coincident signal conditions.
Compared to other multi-channel techniques, such as RF filter banks and channelized receivers, the present invention provides increased frequency measurement resolution using intrachannel phase-based techniques, with sustainable accuracy across the entire operating bandwidth and temperature range through continuous phase offset correction. Current frequency discriminator and IFM devices must operate with a saturated input and therefore function (internally) over a limited amplitude dynamic range to attain accurate measurements. In contrast, the present invention functions correctly over a much wider dynamic range, limited principally by noise and photodetector saturation. This extends the achievable system dynamic range and eliminates RF limiting circuitry, resulting in reduced system cost and improved performance.
Additionally, coherent (IF output) or incoherent (video output) signal detection can be utilized in the present invention. Incoherent or power detection simplifies subsequent signal processing hardware requirements, operating at relatively narrow video bandwidth. In contrast, coherent or heterodyne detection requires more complex processing hardware operating at intermediate frequencies (xe2x80x9cIFxe2x80x9d), but, in principle, can achieve a larger dynamic range. A typical coherent approach requires optical carrier mixing and filtering operations to convert the RF modulated optical photodetector output to a corresponding electrical IF signal, which must then be time-domain processed to measure frequency. The preferred embodiment discussed herein utilizes incoherent signal detection methods to illustrate a relatively simple and low cost approach to frequency measurement.